392 



MAGNETIC OBSERVATORY AT 



at its middle point to the centre of 

 magnet, the axes of the rod and bar 

 being parallel ; the free ends of the 

 rod are enclosed in two zinc tubes, 

 at the inner ends of which, where 

 they nearly meet in the centre, and 

 to their upper surface, two hooks 

 ^ are attached; two loops at the ends 

 of the suspension skein are attached 

 to these hooks, the skein passing 

 over a pulley at the point of sus- 

 pension. Towards either end the 

 rod and tubes are connected by a 

 movable clamp, by which the two 

 may be clamped together at any 

 required distance from the centre. 

 It is evident that by elevation of 

 temperature the free ends of the zinc 

 tubes will be approximated to each 

 other by a quantity equal to the 

 difference of the expansion of the 

 1 lengths of zinc and glass that inter- 

 vene between the sliding clamps and 

 the free ends of the tubes, and, con- 

 sequently, that a diminution to the 

 same extent of the distance between 

 the lower ends of the suspension 

 skein will take place. The varia- 

 tion of the interval between the 

 lower ends of the skein corres- 

 ponding with any given variation of 

 temperature may be made to bear 

 any required ratio to the whole inter- 

 val, first, by a due adjustment of the 

 upper and lower intervals of the 

 skein, and secondly, by varying the 

 position of the sliding clamps ; that 

 is, of the acting lengths of the ex- 

 panding tubes ; the former may be 

 considered as a coarse, the latter as 

 8 a fine adjustment. The glass rod 

 rests on rollers attached to the under 

 surface of the tubes opposite to the 

 hooks, in order that no jerking may 

 be occasioned by the expansion or 

 contraction of the zinc tubes. By 

 these means the quantity b in the 

 preceding formula may be made to 

 vary by change of temperature proportionably to the quantity m with 

 any required degree of exactness; so far, at least, as the variation of 

 m is directly proportional to the variation of temperature. 



